Over the years, a number of devices and techniques have been proposed to provide a greater degree of assurance that a sterilizer is operating effectively to exclude air which could prevent direct contact of steam with the objects being sterilized. In some cases, these have been adopted by hospitals convinced that they needed more reliability.
Efforts to mechanically assure the complete removal of air from steam sterilizers have not been fully successful. Prior to the publication of the Bowie and Dick test in 1963, co-developer J. Dick had employed thermocouples in a simpler form of the test, one in a test package and another at the sterilizer drain. With most loads like the towel package, however, it is not possible to predict the location of the air bubble with the certainty required. The published version of the test replaced the thermocouple with a heat-sensitive tape laid down in two strips in the shape of a St. Andrew's cross to better indicate the extent of air incorporation, but still required monitoring drain temperatures.
In U.S. Pat. No. 3,982,893, Joslyn provides an electronic device that is used with loads of uncontrolled size and character, contrary to the Bowie and Dick protocol which requires the test to be performed on an empty chamber. The Joslyn ('893) device does not provide a controlled challenge to the passage of steam to assure condensation of a sufficient amount to leave behind a detectable amount of air and provides no means for determining if an air bubble exists or might be located. In packs, of the type shown by Joslyn ('893) it is simply not possible to predict where an air bubble might reside. This can be a fatal error.
In U.S. Pat. No. 3,967,494, Joslyn disclosed a method and apparatus for detecting entrapped air in a steam sterilizer. The device is shown outside the steam chamber to enable quantitative measurement of the amount of air or other non-condensable gas at the chamber drain. The device is intended for use with a sterilizer of the type that employs steam to displace air. The device does not, however, eliminate the need for a Bowie and Dick test.
In U.S. Pat. No. 4,309,381 and 4,372,916, Chamberlain and Cook monitor the temperature of the exhausted steam at the drain and the pressure within the chamber, and compare these to the steam table values to determine if all the air has been evacuated by concluding somehow that if fully saturated steam is present no air can be in the chamber. Chamberlain fails to recognize that air in an enclosed chamber with steam will have the same pressure as steam and the same temperature as steam (though not the latent heat energy of steam) and that water droplets can be entrained in fully saturated steam. Thus, this patent cannot achieve the results of the present invention which provides the objective results which Dr. Bowie stated were needed but not available.
In U.S. Pat. No. 4,115,068, Joslyn describes a small device for indicating air inside a sterilization chamber of the type used with steam or ethylene oxide. The device includes an upright tube holding a thermo-sensitive indicator strip and a heat sink. Air is said to pass into the tube where it interferes with the steam contacting the thermo-sensitive indicator strip.
Dyke and Oshlag comment on this arrangement in U.S. Pat. No. 4,594,223, indicating that steam, which is less dense than air, is required to work against gravity, the weight of any accumulated air and the downward moving condensate. They also comment that the condensate forms in the same chamber in which the indicator strip is located and can contact the strip so heat from incoming steam can vaporize the condensate on the strip, thereby interfering with the results. To correct this, they describe a device having a depending glass chamber which permits air to settle to the bottom along with water from condensed steam.
The Dyke and Oshlag device, like that of Joslyn, is simply a replacement for the Bowie and Dick test and requires interpretation of the test results--a source of frequent errors in the original Bowie and Dick test. This is true also of a related device disclosed by Augurt in U.S. Pat. No. 5,066,464, which has a strip of heat-sensitive material in a horizontally-disposed chamber. Similarly, U.S. Pat. No. 4,486,387 to Augurt and U.S. Pat. No. 4,596,696 to Scoville, describe disposable test packages which have Bowie and Dick type test sheets which require interpretation.
In U.S. Pat. Nos. 4,486,387 and 5,066,464, both to Augurt, a requirement is described to provide a challenge for the two most common flaws: 1) an inadequate initial vacuum that leaves air within the packs; 2) air leaks in the chamber, vacuum and/or steam system that permits reentrainment of air in the packs. Those inventions offer a compromise.
There remains a need for a method and apparatus to more objectively, reliably, and precisely determine if a sterilizer is working correctly and to monitor its operation. There is especially a need for a test which provides a positive indication of problems or the absence of them and decreases the probability that either (1) the test performance or (2) interpretation of the results will be dependent upon the skills of a particular operator--both, areas of concern in Bowie and Dick testing.